ng the upper Sepotuba, in the region of the
rapids, there were sandstones, shales, and clays of Permian age. The
rolling country east of this contained eruptive rocks--a porphyritic
disbase, with zeolite, quartz, and agate of Triassic age. With the
chapadao of the Parecis plateau we came to a land of sand and clay,
dotted with lumps of sandstone and pieces of petrified wood; this,
according to Oliveira, is of Mesozoic age, possibly cretaceous and
similar to the South African formation. There are geologists who
consider it as of Permian age.

At Vilhena we were on a watershed which drained into the Gy-Parana,
which itself runs into the Madeira nearly midway between its sources
and its mouth. A little farther along and northward we again came to
streams running ultimately into the Tapajos; and between them, and
close to them, were streamlets which drained into the Duvida and
Ananas, whose courses and outlets were unknown. This point is part of
the divide between the basins of the Madeira and Tapajos. A singular
topographical feature of the Plan Alto, the great interior sandy
plateau of Brazil, is that at its westernmost end the southward
flowing streams, instead of running into the Paraguay as they do
farther east, form the headwaters of the Guapore, which may, perhaps,
be called the upper main stream of the Madeira. These westernmost
streams from the southern edge of the plateau, therefore, begin by
flowing south; then for a long stretch they flow southwest; then
north, and finally northeast into the Amazon. According to some
exceptionally good geological observers, this is probably due to the
fact that in a remote geologic past the ocean sent in an arm from the
south, between the Plan Alto and what is now the Andean chain. These
rivers then emptied into the Andean Sea. The gradual upheaval of the
soil has resulted in substituting dry land for this arm of the ocean
and in reversing the course of what is now the Madeira, just as,
according to these geologists, in somewhat familiar fashion the Amazon
has been reversed, it having once been, at least for the upper two
thirds of its course, an affluent of the Andean Sea.

From Vilhena we travelled in a generally northward direction. For a
few leagues we went across the chapadao, the sands or clays of the
nearly level upland plateau, grassy or covered with thin, stunted
forest, the same type of country that had been predominant ever since
we ascended the Parecis table-land o